Water produced from crude oil and natural gas production usually contains varying residual concentrations of crude oil, natural gas condensates and solids. Water and oil mixtures also result from the water washing of refined petroleum fractions following generally known refining processes, as well as from drainage from the various equipment used in such processes. These contaminants must be removed before the water suspension component can be either used in secondary recovery operations or safely discarded.
The small particles of oil are suspended in the water and held there by mechanical, chemical and electrical forces. The amount of oil contained in the untreated produced water in most systems will vary from about 5 parts per million to about 2,000 parts per million. In some systems, oil contents as high as 20,000 parts per million (2%) have been observed.
The oil droplets in untreated produced water will usually vary in size from 1 to about 1,000 micrometers with most of the oil droplets ranging between 10 and 100 micrometers in diameter. Various methods have been suggested for use in removing oil from produced water, based upon one or more of the following principles: gravity separation of lighter oil droplets from the water; coalescence of the smaller oil droplets; or gas flotation of the oil droplets. The degree of oil removal that is obtained by gravity separation alone typically is inadequate to obtain the necessary oil removal and secondary treatment (such as gas flotation) is required.
Of the above-described principles, gas flotation is the only one which does not rely on gravity separation of the droplets, the gas flotation action in fact being independent of oil droplet size. In gas flotation units, large quantities of gas bubbles are introduced into a flotation chamber. These bubbles float the oil droplets and solids suspended in the oily water to the water's surface, thereby producing an effluent water of substantially reduced oil content.
Two distinct types of flotation units are commonly used, which are distinguished by the method employed in producing the gas bubbles needed to contact the water. These are the dissolved-gas units and dispersed-gas units. U.S. Pat. No. 3,884,803 issued to Traylor describes both types of flotation units. Dissolved-gas units take a portion of the partially treated water effluent and saturate the water effluent with a manufactured gas, such as natural gas or air, by elevating the pressure in a contactor so that the gas is dissolved in the water. The higher the pressure the partially treated water is subjected to, the more gas can be dissolved in the water. Generally, dissolved gas units utilize a contactor pressure of about 20 to 40 psi, with about 20% to 50% of the partially treated water recirculated for contact with the gas. The gas-saturated water is then introduced into a flotation chamber maintained at atmospheric pressure or at a pressure lower than the contactor pressure where the gas breaks out of the solution (i.e., effervesces) so that small diameter bubbles float the oil and solid droplets to the surface where they are mechanically separated.
In the dispersed-gas units, gas bubbles are mechanically dispersed in the oily water either through the use of an inductor device or by a vortex set up by mechanical rotors. Most dispersed-gas units contain three or four cells, where oily water flows in series from one cell to another cell by under flow baffles. In each cell, a portion of the oil and solid droplets are floated to the top for mechanical separation from the treated water.
There are several disadvantages inherent in both the dissolved-gas and dispersed-gas flotation systems. The first is that both systems rely on a recycling of the produced gas, which leads to the gas eventually coming to equilibrium with the water, making it unavailable for use in volatile hydrocarbon stripping. In the dispersed-gas system there is the additional problem associated with the use of high shear pumps or mixers which tend to shear oil droplets. Mixers or pumps also create a mixed cell which prevents having a "quiet" zone for the separation of the oil and water.
Another type of flotation unit is shown in U.S. Pat. Nos. 4,627,922 and 4,752,399 both issued to Viator et al. The unit in Viator et al. first disperses a combination of two gases in an oily water stream at line pressure with turbulent gas dispersion techniques. Then the gas-containing oily water is tangentially injected into a flotation chamber where gas bubbles laden with oil float to the surface of the flotation chamber. The first gas is used to attract the oil droplets but because the bubbles created are very small, a second more buoyant gas is added to impart greater buoyancy to the bubbles to promote movement of the bubbles containing the oil to the surface of the flotation chamber. When the gas-containing water is tangentially injected into the flotation chamber only a portion of the oil contamination has been removed from the water therefore a second combination of two gases is injected at the bottom of the flotation chamber. The second combination of bubbles float upward against the downward swirling water under turbulent conditions to create multistages of contact between the bubbles and water to remove the remaining dissolved oil from the water. The oily water at the bottom of the flotation tank is forced by hydrostatic head through a conduit up to the top of the flotation chamber where it is removed. The two gases are collected, cleaned and recycled for use again in the system.
U.S. Pat. No. 4,198,300 issued to Williams discloses a method and apparatus for removing suspended oil from an oily waste water stream produced from an offshore well. The oily water is injected into a submerged vertical pipe near the upper middle portion of the pipe. Gas produced from the well is injected into the lower end of the pipe and forms bubbles that rise through the pipe countercurrently contacting the suspended oil in the oily water, thereby promoting the separation of the droplets from the oily water. The pipe is unpressurized so that as the bubbles rise through the pipe they are subjected to a reduced hydrostatic pressure which causes the bubbles to grow in size. The growth of the bubbles reduces their effectiveness in removing the small oil droplets from the oily water.
In view of the limitations of the known devices, it is an object of the present invention to provide a means for treating produced oily water which does not rely on high shear pumps or gas saturated water to produce gas bubbles. It is a further object to use a pressurized system so that the size of the gas bubbles can be controlled. It is a still further object of this invention to develop a system for generating larger overall diameter gas bubbles (e.g., 500 micrometers) relative to the size of the oil droplets (e.g., 10 micrometers) and in which the produced gas is not recycled through the system. It is an advantage of the present invention that the gas induced into the system is preferably used only once, and then recovered for alternative use because the gas does not reach equilibrium with the water. The gas can be captured and reused in the present invention.